1. Field of the Invention:
The present invention relates to a process for producing a sintered hard metal body and, in particular, to a sintered hard metal body composed of at least one hard substance from the group including carbides, nitrides and/or carbonitrides of the transition metals of Groups IVB, VB and/or VIB of the Periodic Table of Elements and at least one binder metal from the group including iron, nickel and cobalt, with the at least one hard substance being present as a carbide and/or mixed carbide, and/or a carbonitride and/or mixed carbonitride, and/or a nitride and/or mixed nitride in the form of cubic crystals, in which the powdered starting materials are subjected to mixing, grinding, compressing, and subsequently to sintering. The invention also relates to a sintered hard metal body produced by the process according to the invention.
2. Description of the Related Art:
Processes and compositions for producing sintered hard metal bodies are disclosed, in principle, for example, in Kieffer-Benesovsky, Hartmetall [Hard Metal], Springer-Verlag Pub. (1965), and in Hartmetall fur den Praktiker. Aufbau. Herstellung, Eiqenschaften und industrielle Anwendung einer modernen Werkstoffgruppe [Hard Metals for Practical Structure, Manufacture, Characteristics and Industrial Uses For a Modern Group of Materials], VDI-Verlag GmbH Pub. (1988). In particular, it is known that the useful content of binder metal lies between 3 and 30 weight percent.
Sintered hard metals are known which are based on the hard substances titanium carbide, as described in U.S. Pat. No. 2,967,349, and titanium carbonitride, as described in AT-PS 299,561 and U.S. Pat. No. 3,994,692, bound by means of a nickel-molybdenum binder. These are distinguished by better wear resistance compared to conventional hard metals containing tungsten carbide, as one hard substance phase, cubic titanium mixed carbides, in which part of the titanium atoms are substituted by tantalum, niobium, or tungsten as the second hard substance phase, and cobalt as the binder metal. Titanium carbide and titanium carbonitride hard metals, however, find only limited use as cutting tools, particularly when high cutting speeds are involved and cyclic thermal stresses occur such as during milling. The high temperatures generated at the cutting edges cause the binder metal to lose its strength so that it tends to be plastically deformed under the influence of cutting forces. The noticeably lower thermal conductivity of these TiC--Mo,Ni and Ti(C,N)--Mo,Ni hard metals compared to tungsten carbide undesirably result in accumulation of heat precisely at the point where there is the greatest stress.
To overcome this drawback of TiC--Mo,Ni and the Ti(C,N)-Mo,Ni hard metals, which are superior with respect to wear resistance, it has already been proposed to sinter carbonitride hard substance compositions which include tungsten carbide and an alloyed nickel binder or an alloyed cobalt binder (U.S. Pat. No. 3,840,367 and Federal Republic of Germany Published Application No. 2,546,623, which corresponds to U.S. Pat. No. 4,049,876). However, Ti(C,N) reacts readily with tungsten carbide so that sintering of the hard substance composition must take place under a nitrogen partial pressure which is dependent on the composition and the sintering temperature employed. This, however, undesirably produces microporosity in the structure and causes a reduction in the quality of the hard metal.
U.S. Pat. No. 3,971,656 discloses a hard metal in which the hard substance particles are composed of two phases. The interior of each hard substance particle is composed of a titanium- and nitrogen-rich carbonitride mixed phase and the exterior of each particle is composed of a second phase which is rich in the metals of Group VIB of the Periodic Table of Elements and poor in nitrogen, and which envelops the carbonitride mixed phase comprising the particle's core. Compared to titanium carbide, it is known that titanium nitride increases the resistance to crater formation of hard metals employed as cutting tools for chip cutting work. According to the teaching of U.S. Pat. No. 3,971,656, it is presumed that an equilibrium is established within the hard substance particle composed of two phases. The core of the hard substance particle is thus composed of a carbonitride which is relatively rich in carbon since titanium nitride which is not alloyed is not able to be in equilibrium with the required second phase, which is, for example, a (Mo,W)-rich phase. Thus, the wear resistance of the hard metal, produced according to U.S. Pat. No. 3,971,656 has been determined to be less than optimum.
Another possibility for producing sintered hard metals having improved high temperature resistance is to increase the heat resistance of the binder metal. For example, in addition to including molybdenum in the binder metal, which nickel is able to harden by way of mixed crystal strengthening, aluminum has been additionally alloyed to the binder metal to simulate .gamma.' hardening (hardening due to precipitation of coherent particles having a face centered cubic structure) which is known to characterize superalloys of the binder phase. Electron microscopic examination of aluminum-alloyed binder phases within Ti(C,N)--Mo,Ni hard metals proved the occurrence of .gamma.' phases. The addition of aluminum resulted in an increase of hardness measured at room temperature, however, the hardness increase was accompanied by a decrease in bending strength (see, for example, H. Doi and K. Nishigaki: in Modern Development, Hausner, H. H., Ed., P/M 10, pages 525-542 and D. Moskowitz and M. Humenik, in Modern Development. Hauser, H. H., Ed., P/M 14, page 307, (1980)).
In the process under discussion, the aluminum was added to the hard metal starting mixture in the form of powdered, i.e., very fine grained, Ni--Al alloys having grain sizes in the .mu.m range. Such alloys, however, are extremely difficult and expensive to produce due to the very high plasticity of intermetallic alloys in the Ni--Al system. To realize optimum characteristics for the binder metal, it is therefore also necessary to precisely maintain the prescribed carbon content of the sintered alloy so that the quantity of titanium required for coherent precipitation of the .gamma.' phase goes into solution from the hard substance employed. Only if the percentages of the aluminum dissolved in the binder metal and of the titanium are approximately equal, can a noticeable influence on the characteristics of the binder metal be expected. If the titanium content is too high, the .gamma.' precipitation becomes metastable. If no titanium is present, the coherence tension becomes too low, thus causing the hardening effect to decrease beginning at medium temperatures.
In order to improve heat resistance, AlN has been added to the binder metal as disclosed in Federal Republic of Germany Patent No. 2,830,010, which corresponds to U.S. Pat. No. 4,514,224. The AlN is reported to remain in the structure as a dispersed phase which improves hardness. Under sintering conditions, however, AlN does not form mixed crystals with TiC or with TiN, rather, it constitutes a nonmetal hard substance which does not have good wetting characteristics and, if in finely dispersed form, is not resistant to humidity so that it decomposes into Al(OH).sub.3 and NH.sub.3. This has a very disadvantageous effect particularly during grinding with grinding fluids which are not completely free of water.